The invention relates to an oil pump for use on a vehicle or the like, and in particular, to an oil pump which is provided with an unload valve.
Recently, an oil pump which is intended to be used in a power steering apparatus of a vehicle is increasingly finding its application as a source of pressure for other hydraulic instruments. Generally, a duplex pump is used for a combination of power steering apparatus and another instrument. In this instance, a total pump capacity increases, which in turn increases a load on the pump as compared with the prior art practice, causing an increase in the input power and a reduction in the output from the vehicle. Another disadvantage relates to an increased heating effect. To accommodate for this, there is provided an unload valve which unloads one pump section of the duplex pump which is used for purpose other than driving the power steering apparatus under a given condition.
However, in a conventional oil pump which is provided with an unload valve, the pump and the unload valve are separate from each other, or must be connected together through a piping with consequences that the reliability is degraded as a result of an increased number of interconnections and an oil leakage, that a larger space is required for the assembly and that an increased cost results from an increased number of assembling steps. Additionally, vibration and noise problems arise as a result of a surge pressure which is developed within a lower pressure piping during an unload phase.